Underwater excavation apparatus are known. WO 98/027286, also by the present Inventors comprises an underwater excavation apparatus comprising a hollow body having at least two inlets and at least one outlet, at least one pair of impellers rotatably mounted in the hollow body, and means for driving the impellers, wherein the at least two inlets are substantially symmetrically disposed around an axis extending from the at least one outlet. Therein, preferably, the driving means cause the impellers to be driven in contra-rotating directions, and one of the impellers is provided within one of the inlets and another of the impellers is provided within another of the inlets.
Excavators or mass flow excavation systems operate by directing a jet of high volume fluid under low pressure at the sea bed to displace sea bed material. The excavator is tethered from a vessel by means of a crane wire, which is used to lower and retrieve the excavator, and to maintain a given distance from the sea bed or structure requiring excavation, such as a subsea oil or gas pipeline. In order to control the excavation, sonar detection means are used to allow the excavator operator to view the excavation in real time. Cameras and metal detection means can also be used to assist the operator.
During the excavation of a pipeline or cable the excavator is typically required to follow the direction of the pipeline. This is achieved by moving the vessel along the pipeline, using the dynamic positioning (DP) of the vessel, while the DP operator references the sonar images relayed to surface. In order to follow the line, and to view the excavation process, it is essential that the correct orientation of the excavator is maintained to ensure the sonar is correctly positioned above the line. Present art utilises tugger lines (taut lines attached to the excavator) to maintain and adjust orientation. In inclement weather, wind and tide direction may require the vessel to take a heading different from the direction of the line to be excavated, making it exceedingly difficult to keep the correct orientation of the excavator and the sonar due to the difference in ship's heading and pipeline direction. In subsea excavation apparatus using a single impeller as in U.S. Pat. No. 5,607,289 reactive torque created by the impeller makes the excavation apparatus unstable in the water.
It is an object of at least one embodiment of at least one aspect of the present invention to seek to obviate or at least mitigate one or more of the aforementioned problems in the prior art.
It is a further object of at least one embodiment of at least one aspect of the present invention to seek to obviate or at least mitigate one or more problems in the prior art.